AbstractContext. The physical conditions of the gas forming the
narrow-line regions (NLR) in active galactic nuclei (AGN) have been
extensively studied in the optical band. Recently detailed X-ray
studies have shown how the emission in the 0.1-2 keV band detected in
Seyfert 2 galaxies is associated to gas lying close to or associated
with the NLR. Aims. We take advantage of the spectacular extension (15'') of the NLR in the type II Seyfert galaxy NGC 5252 and of the complementary characteristics of XMM-Newton and Chandra to investigate the physical conditions of the gas in this galaxy. Methods. The X-ray data from XMM-Newton are used to define the spectral properties of the ionising nuclear source. The Chandra
data are used to trace the spatial characteristics of the soft X-ray
emission. This information is then compared to the optical HST
characteristics of the NLR in NGC 5252. Results. The X-ray spectrum of the nucleus of NGC 5252 is intrinsically flat (
-1.5) and absorbed by neutral gas with a column density
cm-2. Below 1 keV a soft excess is detected. The high-resolution spectrum obtained with the XMM-Newton
RGS shows emission lines in the 0.2-1.5 keV range which strongly
indicate that the soft X-ray component is essentially due to ionised
gas. Moreover, the soft X-ray emission is spatially resolved around the
nucleus and well overlaps the images obtained in narrow optical bands
centred around the [O III] emission line at 5007 .
The [O III]/soft-X
flux ratios along the ionisation cones are basically constant. This
indicates that the electron density does not significantly deviate from
the r-2 law (constant ionisation parameter) moving outward from the nucleus. Conclusions. This result combined with previous optical studies
suggests two plausible but different scenarios in the reconstruction of
the last 30 000
years of history of the central AGN. The most promising one is that the
source is indeed a ``quasar relic'' with a steady and inefficient
energy release from the accretion of matter onto the central
super-massive black-hole. This scenario is suggested also by the flat
nuclear X-ray spectrum that evokes an advection dominated accretion
flow (ADAF) like emission mechanism.

1 Introduction

The obscuration of the nuclear
emission in type II AGN allows the study of soft X-ray spectral
components, which are normally outshone by the direct component
in type I unobscured objects. It
has been well known since the early days of X-ray spectroscopy that
excess emission above the extrapolation of the absorbed nuclear
radiation is present in almost all bright Seyfert 2s (Turner et al.
1997). This excess appears smooth when measured with instruments with moderate
energy resolutions like CCD.
However, high-resolution (grating) measurements with Chandra and
XMM-Newton revealed that this excess is generally due to a blending of strong
recombination lines from He- and H-like transitions of elements from carbon to nitrogen (Sako et al. 2000; Sambruna et al. 2001; Kinkhabwala et al. 2002; Armentrout et al. 2007). X-ray spectral diagnostics (Kinkhabwala et al. 2002; Guainazzi & Bianchi 2006) and a close morphological coincidence between the soft X-rays and the [O III] in extended narrow-line regions (ENLR; Bianchi et al. 2006, 2010) strongly indicate that the gas is photoionised by the AGN, with an important role played by resonant scattering.

In this context, NGC 5252 represents an extraordinary laboratory
to study the feedback between the AGN output and circumnuclear gas on
kpc scale because of its spectacular ionisation cones (Tadhunter &
Tsvetanov 1989).

The source NGC 5252 is classified as a Seyfert 1.9
(Osterbrock & Martel 1993) S0 (de Vaucoulers et al. 1991) nearby (z=0.023,) galaxy (
cm-2, Dickey & Lockman 1990). Small
radio jets ()
have been detected and found to be aligned with the ionisation cones
(Wilson & Tsvetanov 1994). Nonetheless, the host galaxy luminosity (
,
Capetti et al. 2005), mass (
,
Marconi & Hunt 2003) and the mass of the central super-massive black-hole
(
,
Capetti et al. 2005 are more typical of quasar than Seyfert galaxies. These pieces of evidence led Capetti et al. 2005)
to speculate that NGC 5252 is most probably to be considered a QSO
relic. This agrees with ``downsizing'' scenarios about the evolution of
super-massive black-hole (SMBH) in cosmic times. According to these
scenarios, most massive SMBHs formed and evolved earlier than lower
mass ones.

Ionisation cones are one of the strongest arguments in favour of the Seyfert unification scenarios (Antonucci 1993).
For this reason, NGC 5252 is also an important laboratory to test
AGN geometrical models. From a diferent point of view, AGN activity has
been recognized, for a while as a key component of the SMBH host galaxy
co-evolution and AGN feedback is likely to self-regulate or be
responsible for the observed properties (Menci et al. 2004).
The very existence of ionisation cones is a witness that feedback/winds
are or were active and thus these sources are ideal laboratories for
feedback.

X-ray measurements allow us to directly link the
properties of the gas emitting optical lines with the intrinsic AGN
power, which in type II AGN can be truly measured only at energies
higher than the soft photoelectric cut-off due to the AGN obscuring
matter. Furthermore, the morphological coincidence between X-rays and
optical emission in ENLR (Bianchi et al. 2006) points to a fundamental
physical link between the two wavebands. They need to be studied
simultaneously to derive the correct energy budget in the
ionisation cones. Prompted by these motivations, we have performed deep
X-ray observations of NGC 5252 at the highest spatial and spectral
resolution currently available with Chandra and XMM-Newton. The
results of these observations are the subject of this paper.

2 The nuclear spectrum

NGC 5252 was observed by XMM-Newton on 2003, July 18th,
with the EPIC CCDs (MOS and pn in full window, see Table 1) as the prime instrument
for a total duration of 67 ks. The
Observation Data Files (ODFs) were reduced and analysed with the
latest Science Analysis System (SAS) software package
(Gabriel et al. 2003) with the associated latest
calibration files. We used the epproc and emproc tasks to
generate event files and remove dead and hot pixels.
Several time intervals with a high background rate were
identified in the
single events light curve at energies >10 keV and were
removed, yielding a net exposure of 50 ks for the MOS
and 38 ks for the pn. Pile-up is negligible in this source
according to the epatplot SAS task outcome.
Patterns 12 and 4 were considered for MOS and
pn, respectively. The source counts were extracted from a circular region
with radius 50
,
thus encompassing a large
fraction of the optically-defined galaxy. The background was
estimated with both
blank-sky files and locally from a offset source-free region.
Light curves in the soft (0.5-2 keV) and hard (2-10 keV) energy bands
were first investigated. We found no significant flux nor spectral
variations, thus considered the time averaged spectrum.

If the nuclear emission is modelled with a simple absorbed power-law we
obtain an extremely flat photon index [
and
cm-2, the reported errors
are, here and hereafter, at 90% confidence level],
plus a soft component emerging at energies below keV. It can be
parameterised with a scattered power-law with a steep photon index
.
There is also evidence for
one (or few) soft emission lines in addition to the soft
continuum, a Fe K
line with
keV and
eV, and an absorption edge
at
keV and optical depth
.
A significantly better fit is obtained for a power-law plus a thermal
component, namely mekal in Xspec (Mewe et al. 1985),
which is used to model the soft X-ray band
of NGC 5252 (/d.o.f. = 1436/1410 in the first case while
/d.o.f. = 1329/1410 with mekal).
In this case, the temperature of the plasma
is
keV.
The investigation on the true nature of
this soft X-ray component will be the subject of the next sections.
Here it is important to note that whatever the fitting of the data below 1 keV, the best-fit
model for the nuclear emission of NGC 5252 above 1 keV
is typical in shape, but flatter (
-1.5) than normally
found from Seyfert 2 galaxies (
-2.5, Turner & Pounds 1989;
Turner et al. 1997; Risaliti 2002; Cappi et al. 2006;
Dadina 2008). It is however
consistent with the previous ASCA measurement (
-1.5,
Cappi et al. 1996), confirming the need in this source for a more complex
absorption (either multiple, ionised or both) to recover a steeper
canonical photon index. With the above model we measure for the soft (0.5-2 keV) component a flux of
erg cm-2 s-1 corresponding to a luminosity of
erg/s and for the hard (2-10 keV) component a flux of
erg cm-2 s-1 corresponding to a (unabsorbed)
luminosity of
erg/s. This is consistent, within a few tens
of percent, with previous ASCA and BeppoSAX values (Cappi et al. 1996; Dadina 2007). It is worth noting here that from IR diagnostics we expect that star-forming activity should contribute less than 1% to the total soft X-ray emission (Cappi et al. 1996).

3 High-resolution spectroscopy of the AGN environment

Figure 2:

Chandra ACIS-S images of NGC 5252 in the 0.1-1 keV (soft; left panel), and 1-10 keV (hard; right panel). The images were smoothed with a
pixel (yielding an angular resolution of 1''
in the images) wavelet for illustration purposes. The thin solid lines
represent nine linearly spaced contours in the range of 2 to 20 counts
per pixel. The thick dot-dashed line indicates the position of the
out-of-time events readout streak, removed before the generation of the
image. The dashed lines represents the regions whence the spectra of
the south and north diffuse spots were extracted. The solid circles
represent the regions whence the spectra of the nucleus (the brightest
and central spot) and of the south-east nuclear source were extracted.

Table 1:
Main characteristics of the X-ray observations presented here.

The RGS data were reduced starting from the Observation Data Files
with SASv6.5 (Gabriel et al. 2003) and using the latest calibration files.
The SAS meta-task rgsproc was used
to generate source and background spectra, assuming as a reference
coordinate coincident with the optical nucleus of NGC 5252. Background
spectra were generated with both blank field maps - accumulated across
the whole mission - and a ``local'' background accumulated during the
observation. The former overestimates the intrinsic background level during the
observation and is based on a model of the estimated background on
the basis of the count rates detected in the most external of the camera
CCDs. We therefore employed the ``local'' background hereafter.
A correction factor to the count background spectrum was applied to take
the size of the extraction region into account, which corresponds to the area
of the RGS active CCDs outside the 98% percentage point of the line
spread function in the cross-dispersion direction.

Figure 3:

Radial profile (filled
circles) of the ACIS-S hard-band image. The solid line represents the
PSF for a source with the same hard X-ray spectral energy distribution
as the NGC 5252 ``nucleus normalized'' to its on-axis peak flux.
When not visible, the error bars are within the filled circles.

We simultaneously fitted the spectra of the two cameras following the procedure outlined in Guainazzi & Bianchi (2006) who performed local spectral fits around each of the 40 emission lines detected in the archetypal obscured Seyfert NGC 1068 (Kinkhabwala et al. 2002).
In these fits, both the background level and the continuum were assumed
as independent power-law components with the photon index
set equal to 1. It is worth noting that the adopted value of the power
law index, here equal to the photon index of the continuum of the
primary emission, does not affect the results signifcantly, given the
very limited band of these fits. Different choices for the continuum
spectral index yield indistinguishable results. Each emission line was
modelled with an unresolved Gaussian profile fixed to be at the
expected energies (leaving the intrinsic width of the profile free
yields a negligible improvement in the quality of the fit). We detect
three lines (see lower panel of Fig. 1) at a confidence level larger
than 90% [
,
24.0, 10.8 for CV, OVII and OVIII lines, respectively, for one interesting parameter (Table 2)].

Diagnostic parameters involving the intensity of the O VII He-triplets can be used in principle to pinpoint the physical process responsible
for the bulk of the X-ray emission in high-resolution spectra. The detection
of the forbidden (f) component only allows us to set lower limits
on the standard triplet diagnostics (Gabriel & Jordan 1969; Porquet & Dubau 2000):
R > 1.1, G > 0.7 (where R is the ratio between forbidden and
intercobination lines and depends on the electron density, while G is the
ratio between intercombination plus forbidden lines and the resonance line, Gabriel & Jordan 1969; Porquet & Dubau 2000).

These limits, although fully consistent with
photoionised plasmas, do not rule out collisional ionisation. Guainazzi &
Bianchi (2007) proposed a criterion to distinguish on a statistical
basis between AGN- and starburst-powered sources based on the location
of the source in an empirical observable plane: integrated luminosity of the
He- and H-like oxygen lines, ,
against the intensity ratio
between the f and the O VIII Ly-.
In NGC 5252
,
and
erg s-1.
These values put NGC 5252 in the plane locus preferentially
occupied by photoionised (AGN) sources (Guainazzi et al. 2009). We also estimated the flux density associated to the continuum, using a line-free energy range between 586 and 606 eV:
erg s-1.

4 X-ray imaging of the ionisation cones

Chandra observed NGC 5252 on August 11, 2003 with the ACIS-S
detector in standard VFAINT configuration. Data reduction was performed
with CIAO version 3.3 and associated CALDB version 3.2. ``Level 1''
events were corrected for bad pixels, gain spatial dependency,
and charge transfer inefficiency via acis_process_events.

Although the correction for read-out streaks was applied
as well, some out-of-time events remained in the final cleaned event
list, and were removed by applying a 2 pixels (1
)-wide
tilted rectangular box around the streak.

The ACIS-S images in the
2
around the optical core of
NGC 5252 are shown in Fig. 2 in the 0.2-1 keV and
in the 1-10 keV energy bands. The soft band clearly shows extended emission
in the north-south direction on both sides of the nucleus. On the
contrary, the hard band image is point-like. We extracted a radial
profile of the latter and compared it with the expected instrumental point
spread function (PSF) of a source
with the same spectral energy distribution as the
NGC 5252 nucleus. The two profiles are perfectly consistent
up to 30
off-axis (see Fig. 3).

In order to characterize the spectral behaviour of the diffuse emission,
we extracted spectra from the four regions, identified in the soft image
(Fig. 2): the nucleus (N), a S-E source about 3.2
from the
nucleus (SENS), and the South (SS) and North (NS) diffuse Spots. Background
spectra were generated from a large circle 57
wide around the
galaxy core, once a 21
inner circle, as well as
5
circles around each serendipitous point sources were removed. Alternative
choices of the background regions do not substantially change
the results presented in this section. Source spectra were rebinned
to over-sample the intrinsic instrumental energy resolution by
a factor 3, and to have at least 25 background-subtracted counts
in each spectral bin. The latter criterion
ensures the applicability of the
statistics.

For all spectra we employed a baseline model that include
a thermal emission component from collisionally excited plasma (mekal in
X SPEC; Mewe et al 1985).
This choice was done for simplicity and the only information
obtained using mekal is the flux of the thermal component.
This is particularly true for the SS and NS regions where the complexity of
the mekal model is well above the quality of the data.
Moreover, a photoelectrically-absorbed power-law was
always included in the data.
The physical meaning of the latter varies
depending on the region in which the spectrum was extracted. For the nuclear
region, the non-thermal component represents the contribution of the active
nucleus; for the other regions, the integrated contribution of hard galactic
sources like for example X-ray binaries, cataclysmic variables or
supernova remnants. We therefore
refrain from attributing a physical meaning to the power-law spectral
indeces in the latter case.
The spectra and corresponding best-fits are shown in Fig. 4.
A summary of the spectral results is presented
in Table 3. The Chandra data confirm that the nuclear spectrum

is remarkably flat and is
seen through a substantial column density (
cm-2).

The background subtraction for the spectrum of SENS could be
contaminated by the spilling of the nuclear emission. The encircled
energy fraction at a distance equal to that between sources N and SENS
is 97.5%.
However, subtracting a properly rescaled nuclear spectrum to the SENS
spectrum yields negative counts above 2 keV. In order to have an
independent estimate of the
spectral behaviour of SENS, we extracted images 10
around the nuclear region in narrow energy bands (Fig. 5): 200-400 eV,

Figure 5:

Narrow-band ACIS-S images in the 10
around the NGC 5252 core normalized to the peak nuclear emission. Images are smoothed with a 5 pixel Gaussian kernel.
The solid lines in the upper left panel represent a contour plot of the 0.2-1 keV
image, assuming the same smoothing criterion.

500-600 eV, 600-700 eV and 700-1000 eV. In a
line-dominated plasma, the above energy ranges correspond to bands
dominated by C VI and C V K,
O VII He-,
O VIII Ly-,
and Fe-L transitions, respectively. Each image was normalized to the
peak of the
nuclear emission in that energy band. The soft X-ray SENS spectrum is
comparatively dominated by oxygen transitions, with little contribution
in either the carbon or the iron band.

5 Comparing soft X-ray and [O III] morphologies

NGC 5252 was observed in the [O III] band with the
WFPC2 on-board HST on 1995, July 23, with the linear ramp filter
FR533N. The data were downloaded from MAST (multi-mission archive at
STScI). The images were processed through the standard OTFR (on-the-fly
reprocessing) calibration pipeline which performs analog-to-digital
conversion, bad pixel masking, bias and dark subtraction, flat field
correction and photometric calibration. The cosmic ray rejection was
performed combining the two images that are usually taken for this
scope. Geometric distortion was corrected with the multi drizzle script (Koekemoer et al. 2002).

The relative Chandra-HST astrometry is clearly a fundamental issue for this work. Chandra has a nominal position accuracy of 0.6
while the absolute astrometry of HST is accurate to 1-2
.
Fortunately we could use a point-like source detected both in the WFPC2 and Chandra fields to align the two astrometric solutions. This source was previously detected at radio
wavelengths (Wilson & Tsvetanov 1994) and is most probably associated to a background quasar (Tsvetanov et al. 1996). Moreover we used the brightest emission peak in the nuclear region of NGC 5252 itself as a second reference point.

Images were calibrated in flux with a constant flux conversion
factor of
,
corresponding to the flux producing a
count rate of 1 s-1 in the filter band. The above value is appropriate
for the instrumental configuration employed during the
NGC 5252 exposure, as indicated by the PHOTFLAM keyword in the image file.

Tsvetanov et al. (1996), Morse et al. (1998) and Capetti et al. (2005) discussed it in detail. We refer the reader to these papers for an extensive
discussion on the properties of the optical emission. Their main outcomes can be summarized as follows:

the surface brightness is dominated by the unresolved nucleus;

a half-ring structure is apparent S-E of the nucleus at a
maximum projected distance of 1.5 kpc. It is probably associated
with the near side of an inclined gas disc, whose far side is
obscured by the host galaxy dust (Morse et al. 1998);

the large scale ionisation cone is traced by thin shells of
enhanced emission at either side of the nucleus, well aligned along
a
at distances between 5 and 11 kpc. Fainter
co-aligned structures at scales as large as 20 kpc are
detected as well in the O[ III] images;
however, we will not discuss these latter structures, as
they are beyond the region where X-ray emission associated with
NGC 5252 is detected;

there is no evidence of radial motions. The measured velocities
of the different structures are fully explained by the rotations of the
two discs [nonetheless Acosta-Pulido et al. (1996) claimed the detection
of radial motions describing the kinematic properties of the [O III] emission
arcs].

Figure 7:

Iso-intensity Chandra-ACIS 0.2-1 keV X-ray iso-intensity
contours superposed to the HST WFC 2 [O III] image
of Fig. 6. The resolution of the latter was
degraded to the typical resolution of Chandra optics
by applying a wavelet smoothing with an 8 pixel
kernel. The Chandra contours represent
nine linearly space count levels from 0.5 to 20 counts
per pixel, after a wavelet smoothing with a
pixel was applied.

In Fig. 7 we present
the superposition between the soft X-rays iso-intensity contours to the [O III] image. The HST image spatial resolution was degraded with an 8 pixel wavelet kernel to match the resolution of the Chandra optics. Regions of enhanced X-ray emission exhibit a remarkable coincidence with the
morphology of the optical narrow-band image.

We calculated the ratio between the [O III] band and the 0.5-2 keV flux (Fig. 8) for the regions specified after splitting region SS into two sub-regions divided by a E-W line at
(regions ``SSNorth'' - SSN - and ``SSSouth'' - SSS - respectively). The
ratio exhibits a dynamical range smaller than a factor 2 over distances
ranging from less than 100 pc to 1.5 kpc with a slight tendency to decrease with the distance from the nucleus (r).
This last effect, however, is most probably an observational artifact
due to the decreasing in surface brightness of the arcs moving away
from the nucleus coupled with the sensitivity limits of Chandra to extended sources. At a first glance, the [O III]/soft-X ratio profile as a function of the distance from the nucleus seems to suggest that the electron density follows a r-2
relation because the number of ionising photons and of the overall
average ionisation state of the nuclear species remains almost
constant. This result agrees with Bianchi et al. (2006) who assumed a very simplified geometry of the emitting gas
though. A more detailed investigation on this
topic in NGC 5252 is hampered by the quality of the data.

6 Discussion

The soft X-ray emission of NGC 5252 is clearly
extended and the ACIS images demonstrate that
the spectacular ionisation cones observed in [O III] have counterparts
in the 0.1-1 keV band.
The cumulative soft X-ray spectrum observed by XMM-Newton is described by
a soft power-law (
). The ACIS images suggest that this is probably due
to a blend of emission lines that mimics this steep power-law as demonstrated
in other type II Seyferts like NGC 1068, Circinus galaxy and Mrk 3 (Kinkhabwala et al. 2002; Brinkman et al. 2002; Ogle et al. 2003; Sambruna et al. 2001; Sako et al. 2000; Bianchi et al. 2005; Pounds & Page 2005).
This scenario is also supported by the detection in the RGS high resolution
spectrum of three emission
lines, of CV, OVII and OVIII, probably due to photoionised gas. This is also
consistent with previous
optical studies that excluded
collisional ionisation along the cones of NGC 5252 (Tsvetanov et al. 1996).
Moreover, the presence of in situ ionisation sources due to shocks formed by
large scale outflows interacting with the interstellar matter has been
excluded (Morse et al. 1998).
Thus the source of ionising photons is most probably the nucleus. With this
assumption,
we can use the imaging of the arcs to study the physical condition
of the gas along the ionisation cones. In particular,
the constant of [O III]/(0.5-2 keV) flux ratio along the ionisation cones
within the inner1.5 kpc suggests a r-2 law for the ion density.

Optical spectroscopic studies (Acosta-Pulido et al. 1996) suggest that the
radial dependency of the ionisation parameterU follows a different law in the south-east (
)
with respect to the north-east (
)
cone. The authors speculate that the intrinsic behaviour should be the
one shown in the former, while the radial-independence of the
ionisation parameter in the latter may be due to a ``conspiracy''
introduced by the existence of two counterotating discs of gas (Morse
et al. (1998): one is coplanar to the stellar one and the other is inclined by 40.
Morse et al. (1998) speculated that the prominence of the southeast [O III] cone in the nuclear regions is caused by this component being seen directly, while the northeast [O III] cone is seen through the gas of the other disc.
If that is the case, the absorption due to this component could alter the line ratios
presented by Acosta-Pulido et al. (1996) and thus the correct behaviour of U should be the one derived from the south-east cone.
implies that the luminosity of the nucleus increased by a factor
-6 in the last
years. These numbers become
-30 and
years if we further assume that the U and the ion density laws are
still valid up to 10 kpc from the nucleus, i.e. where the optical
cones are still detectable in [O III] but not in X-rays.

On the contrary, having U constant and
would imply that L has remained constant during the last 5000
(30 000) years. This is consistent with the ``quasar-relic''
scenario proposed by Capetti et al. (2005).
These authors suggested that the nucleus of NGC 5252 is indeed the
``relic'' of a nucleus that already experienced the activity phase in
the
past and that now persists in an almost quiescent phase. This is
suggested by the high mass of the SMBH (
,
Capetti et al. 2005) that indicates that the nucleus has already accreted in the past, the low Eddington ratio (
,
assuming the bolometric
correction from Marconi et al. (2004),
),
and the early type (S0) morphology of the AGN host galaxy. In the
literature it is also reported that the optical emission line ratios in
the inner 30" are typical of LINERS (Goncalves et al. 1998),
thus suggesting that a low efficiency engine is acting at the nucleus
of the source. It is worth noting that also the detection of two
counterotating discs suggests that NGC 5252 is a ``quasar-relic''.
These discs are tracers of a major merging event that occurred most
probably more than 108 years ago, because the stellar disc
of NGC 5252 is undisturbed. If the merging event triggered a phase
of AGN activity (see Jogge 2006, and references therein for a
discussion of this topic), we can expect that it lasted few/some 107 years (Martini & Weinberg 2001; Steidel et al. 2002; Jakobsen et al. 2003; Gonçalves et al. 2008) after which the source has persisted in a quiescent state.

Finally it is worth noting that the spectrum of the nucleus hosted by
NGC 5252 is confirmed to be quite flat (Cappi et al. 1996).
As shown, if modelled with a simple absorbed power-law its photon
index points to a very hard spectrum (
).
The low EW (50 eV) of the neutral (
keV) iron line
is consistent with what is expected if the FeK
line is
produced via transmission in the observed column
(
cm-2, Makishima 1986)
thus excluding a reflection dominated spectrum. To reconcile the
hardness of the NGC 5252 nuclear spectrum at least marginally,
with previous results for Seyfert galaxies (
-2.5;
Turner & Pounds 1989; Nandra & Pounds 1994; Smith & Done 1996; Dadina 2008), we must invoke
complex absorption models involving partial covering of the source and/or
ionised absorbers along the line-of-sight. In this
case the spectral index becomes
-1.5.
It is remarkable that the flat photon index
may be a further clue suggesting that the X-rays may be
produced in an ADAF (Narayan & Yi 1994) as expected in a ``quasar-relic''.

Acknowledgements

This paper is based on observations obtained with XMM-Newton, an ESA
science mission with instruments and contributions directly funded by
ESA Member States and the USA (NASA). M.D. greatfully acknowledges Barbara De
Marco for helpful discussions. M.D., M.C. and G.M. greatfully acknowledge
ASI financial support under contract I/23/05/0. C.V. greatfully acknowledges
ASI financial support under contract I/088/06/0.

Footnotes

... (2006)

This paper discusses a sample of 69 RGS spectra of type 1.5,
1.8, 1.9 and 2 Seyfert galaxies. The observation of NGC 5252
discussed in this paper belongs to this sample as well.

... inner

The outer arcs and filaments (Tadhunter & Tsvetanov 1989) are most probably too weak to be detected in X-rays. Considering the extension of the outer [O III] arcs, the minimum detectable flux between 0.1-1 keV is
erg s-1 cm-2 while assuming a constant [O III]/soft X-ray ratio, the expected flux should be 10 times lower.

... parameter

,
where L is the source's luminosity,
is the density of the ionised gas, r is the distance between the source of ionising photons and the ionised matter.

All Tables

Table 1:
Main characteristics of the X-ray observations presented here.

Chandra ACIS-S images of NGC 5252 in the 0.1-1 keV (soft; left panel), and 1-10 keV (hard; right panel). The images were smoothed with a
pixel (yielding an angular resolution of 1''
in the images) wavelet for illustration purposes. The thin solid lines
represent nine linearly spaced contours in the range of 2 to 20 counts
per pixel. The thick dot-dashed line indicates the position of the
out-of-time events readout streak, removed before the generation of the
image. The dashed lines represents the regions whence the spectra of
the south and north diffuse spots were extracted. The solid circles
represent the regions whence the spectra of the nucleus (the brightest
and central spot) and of the south-east nuclear source were extracted.

Radial profile (filled
circles) of the ACIS-S hard-band image. The solid line represents the
PSF for a source with the same hard X-ray spectral energy distribution
as the NGC 5252 ``nucleus normalized'' to its on-axis peak flux.
When not visible, the error bars are within the filled circles.

Narrow-band ACIS-S images in the 10
around the NGC 5252 core normalized to the peak nuclear emission. Images are smoothed with a 5 pixel Gaussian kernel.
The solid lines in the upper left panel represent a contour plot of the 0.2-1 keV
image, assuming the same smoothing criterion.

Iso-intensity Chandra-ACIS 0.2-1 keV X-ray iso-intensity
contours superposed to the HST WFC 2 [O III] image
of Fig. 6. The resolution of the latter was
degraded to the typical resolution of Chandra optics
by applying a wavelet smoothing with an 8 pixel
kernel. The Chandra contours represent
nine linearly space count levels from 0.5 to 20 counts
per pixel, after a wavelet smoothing with a
pixel was applied.

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